The invention relates to an eddy current sensor with at least one measuring coil that can be supplied with alternating current, and with an evaluation circuit.
Eddy current sensors of the kind under discussion may be used, for example, for measuring a distance, or even for determining the conductivity of a measuring object. Depending on material and quality of the measuring object, an eddy current sensor may be used to determine even other properties of the surface coating of the measuring object, such as, for example, coating thickness. A further field of application for an eddy current sensor is the thickness determination of thin foils.
Eddy current sensors are frequently used in industrial surroundings, since they are also suitable for measurements under problematic environmental conditions, such as, for example, in the presence of strong magnetic fields, at high temperatures, and at a relatively high degree of pollution. However, strong temperature fluctuations also affect measurements with eddy current sensors. The present invention therefore concerns compensation of such temperature influences on measured data that are obtained with the aid of an eddy current sensor.
Temperature fluctuations affect the measurement results of an eddy current in two respects. First, the complex impedance of the measuring coil changes, in particular of the real parts of the complex impedance. Besides that, however, it is also possible to detect even a small temperature dependence of the imaginary part of the complex impedance. Second, the conductivity of the measuring object or the material thereof changes likewise along with the temperature. In certain temperature ranges, there exists an almost linear relationship between the temperature of the measuring object and its conductivity. Since conductivity of the measuring object material becomes effective on the eddy currents that are induced in the material via the measuring coil, the conductivity of the measuring object material influences again the measuring coil or its impedance via the feed back of the coupling. A complete compensation of the temperature influence on the measured data will therefore be possible only when both effects are compensated, i.e., both the temperature influence on the impedance of the measuring coil and the temperature influence on the conductivity of the measuring object material.
International Patent Application PCT/DE 93/00703, Publication WO 94/03778 and corresponding U.S. Pat. No. 5,629,619, disclose a method of compensating temperature influences on the output signal of a measuring coil, wherein a dc voltage is superposed on the ac voltage that supplies the measuring coil. The ac component of the output signal of the measuring coil is influenced both by the measuring object and by temperature influences. Contrary thereto, the dc voltage component of the output signal is subjected only to temperature influences, so that the temperature influences can be determined, isolated via the dc voltage component, and thus be considered and compensated in the evaluation of the dc voltage component of the output signal.
In the known method, the dc voltage component of the output signal of the measuring coil thus assists in only compensating the temperature influence on the real part of the measuring coil impedance. It is thus not possible to eliminate a possible temperature influence on the imaginary part of the measuring coil impedance. The temperature influence on the output signal of the measuring coil, which is due to a temperature dependence on the conductivity of the measuring object, is likewise compensated in this instance only when the measuring coil and the measuring object are subjected to the same temperature. In particular at high temperatures, this condition is often not met.
It is therefore the object of the invention to describe an eddy current sensor of the kind under discussion that facilitates with a simple construction and an evaluation circuit a reliable compensation of temperature influences both on the real part and on the imaginary part of the impedance of the measuring coil.